Studies on new procedures for bone reconstruction suggest that autologous cells seeded on a resorbable scaffold can improve the treatment of bone defects. It is important to develop culture conditions for ex vivo expansion of stromal stem cells (SSC) that do not compromise their self-renewing and differentiation capability. Bone marrow SSC and platelet gel (PG) obtained by platelet-rich plasma provide an invaluable source for autologous progenitor cells and growth factors for bone reconstruction. In this study the effect of platelet-rich plasma (PRP) released by PG on SSC proliferation and differentiation was investigated. MTT assay was used to investigate the effect of PRP on proliferation: results showed that PRP induced SSC proliferation. The effect was dose dependent and 10% PRP is sufficient to induce a marked cell proliferation. Untreated cells served as controls. Upon treatment with 10% PRP, cells entered logarithmic growth. Removal of PRP restored the characteristic proliferation rate. Because SSC can gradually lose their capability to differentiate along the chondrogenic and osteogenic lineage during subculture in vitro, we tested whether 10% PRP treatment affected SSC ability to mineralize. SSC were first exposed to 10% PRP for five passages, at passage 6 PRP was washed away and plated cells were treated with dexamethasone (DEX). DEX induced a three-fold increase in the number of alkaline phosphatase positive cells and induced mineralization that is consistent with the differentiation of osteochondroprogenitor cells. In conclusion, 10% PRP promotes SSC proliferation; cells expanded with 10% PRP can mineralize the extracellular matrix once PRP is withdrawn.